1. Field of the Invention
This invention relates to preheating oxidant for a fuel cell power generator within the generator itself, simplifying the generator design and reducing costs by eliminating expensive high temperature metal manifolding and improving start-up methodology.
2. Description of the Prior Art
Fuel cell based, electrical generator apparatus utilizing solid oxide electrolyte fuel cells (“SOFC”) arranged within a housing and surrounded by insulation are well known, and taught, for example, by U.S. Pat. No. 4,395,468 (Isenberg) and “Solid Oxide Fuel Cell”, Westinghouse Electric Corporation, October 1992, for tubular SOFC. The tubular type fuel cells can comprise closed ended, axially elongated ceramic tube air electrode material, completely covered by thin film, ceramic, solid electrolyte material. The electrolyte layer is covered by cermet fuel electrode material, except for a thin, axially elongated, interconnection material.
Development studies of SOFC power plant systems have indicated the desirability of pressurized operations. This would permit operation with a coal gasifier as the fuel supply and/or use of a gas turbine generator as a bottoming cycle. Integration is thought commercially possible because of the closely matched thermodynamic conditions of the SOFC module output exhaust flow and the gas turbine inlet flow. One such pressurized system is described in U.S. Pat. No. 3,972,731 (Bloomfield et al.).
Usually the air/oxidant is externally pre-heated, as taught in U.S. Pat. No. 5,413,879 (Domeracki et al.), and also internally heated, by passing the air through a combustion chamber before entry into the interior of the fuel cell, as taught by Zafred et al. in FIG. 4 of U.S. Pat. No. 5,573,867. This is also shown in U.S. Pat. No. 4,664,986 (Draper et al.) where metal finned inserts were used within the air/oxidant feed conduits, within the combustion chamber/pre-heater section, to increase heat transfer.
Tubular ceramic solid oxide fuel cell generators operate in the temperature range 800° C. through 1000° C. Because the cells are of ceramic construction substantial temperature gradients must be avoided in order to prevent cracking. Consequently, the air which is delivered to the cells must be preheated to a temperature which is close to that of the generator operating temperature. The delivery of air at a temperature in the region of 700° C. is also essential from the viewpoint that the cell temperature must be maintained at a high enough level that the cells remain electrochemically active over their entire length.
When the means of preheating air is external to the generator the conduits and plena that are used to deliver the air to the generator must be constructed from refractory materials. High temperature metal alloys such as Inconel 601 or Inconel 617 (both Ni, Cr based metal alloys), and high alumina ceramics have consistently been the materials of choice. These materials are extremely expensive and in some cases they are difficult to fabricate.
Another difficulty with present practice is that the high operating temperature of the air delivery components precludes the creation of very effective seals such as might be feasible if the use of elastomers were possible. Consequently, a fraction of the air which is pumped by the blower leaks from the air plenum directly to the exhaust plenum and does not pass through the generator. This situation is wasteful of energy.
What is needed is an air preheating system which is an integral part of the generator so that the conduits and plena that are used to convey the air to the generator can be constructed from more conventional materials such as the common steels, plastics or fiberglass. Ducting would not need to be insulated. Furthermore, it is preferable that elements of the air delivery system can be very effectively sealed to prevent leakage of air from the air feed plenum to the exhaust plenum, thus ensuring that all of the air which is pumped has value in the electrochemical and thermal scheme. The existence of an integral air preheater which ensures the delivery of cool air to the module facilitates this objective.
What is also needed is a means of start-up heating which does not compromise the above stated advantages of an integral preheater. Preferably the start-up heater would serve also as a power dissipater to the SOFC generator, so that when the generator is off-line it can continue to operate at reduced power level in a self-heating mode. When the SOFC bundle, air preheater and start-up/power dissipater heating means is packaged in such a way as to promote modular constructions, fabrication cost can be reduced and inherent size scalability is promoted.